Induction charging systems are well known in the field of portable electrical devices. For example, portable motorized toothbrushes typically contain a rechargeable battery which is charged by induction. Similarly, portable wireless communication devices, such as two-way RF radios, cellular phones, paging devices, and wireless communicators, commonly utilize a rechargeable battery that, in certain applications, is recharged by contactless induction charging. Such portable devices are becoming increasingly popular because of the convenience afforded a user by working without a wired connection, such as not having to connect plugs to sockets, not having to precisely locate and plug a unit to be charged, and the ability to quickly remove from a charger unit a device that has been recharged.
Unfortunately, present induction charging systems either have little control over input voltage to the charge control circuitry, or such systems employ wireless techniques to control these parameters by regulating the base of the charging system. Notably, implementation of wireless control techniques within the charging system is expensive. Moreover, regulating the base to control charge parameters oftentimes limits the charging system to efficient charging of only one device at a time. If such a base were to be used to charge two or more devices requiring different charge voltages, charge efficiency would suffer. For instance, the base may optimize the primary voltage for only one of the devices, such as the device requiring the highest charge voltage. Such a voltage level can damage the battery cell of a device requiring a lower charge voltage. A voltage drop network could be incorporated into the latter device in order to achieve the optimum charge voltage, but the energy dissipated by the voltage drop network would be wasted. More particularly, the energy would be transformed into heat, which can be harmful to battery cells, operation of the charging device, or operation of the battery operated device. Accordingly, what is needed is a relatively inexpensive induction charging system which provides the capability of simultaneously, and efficiently, charging multiple devices.